U.S. patent application number 10/971110 was filed with the patent office on 2005-05-26 for sulfur purge control method and exhaust gas purifying system.
This patent application is currently assigned to Isuzu Motors Limited. Invention is credited to Gabe, Masashi, Nagaoka, Daiji.
Application Number | 20050112046 10/971110 |
Document ID | / |
Family ID | 34431280 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050112046 |
Kind Code |
A1 |
Nagaoka, Daiji ; et
al. |
May 26, 2005 |
Sulfur purge control method and exhaust gas purifying system
Abstract
In an exhaust gas purifying system (1) for removing NOx by a NOx
occlusion reduction type catalyst (11) for exhaust gas of an
internal combustion engine, the air-fuel ratio in the exhaust gas
is controlled by setting the target air-fuel ratio at a
predetermined first air-fuel ratio which is a rich air-fuel ratio
by a sulfur purge control means (C24) after the start of sulfur
purge, and thereafter, the air-fuel ratio in the exhaust gas is
controlled by changing the target air-fuel ratio to the
predetermined second air-fuel ratio which is a stoichiometric
air-fuel ratio, when the oxygen concentration (Od) in the
downstream of the NOx occlusion reduction type catalyst (11)
measured by an oxygen concentration detection means (C12) becomes
lower than a predetermined threshold. Thereby, sulfur component
accumulated in the NOx occlusion reduction type catalyst (11) can
be purged efficiently, while preventing carbon monoxide from being
discharged into the atmospheric air.
Inventors: |
Nagaoka, Daiji;
(Fujisawa-shi, JP) ; Gabe, Masashi; (Fujisawa-shi,
JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Isuzu Motors Limited
Tokyo
JP
|
Family ID: |
34431280 |
Appl. No.: |
10/971110 |
Filed: |
October 25, 2004 |
Current U.S.
Class: |
423/239.1 ;
422/177; 423/213.2; 60/274; 60/277; 60/284 |
Current CPC
Class: |
F01N 3/085 20130101;
Y02A 50/20 20180101; Y02T 10/40 20130101; Y02T 10/47 20130101; F01N
9/00 20130101; Y02A 50/2344 20180101; F02D 41/405 20130101; F01N
3/0814 20130101; F01N 2260/04 20130101; F01N 2570/14 20130101; F01N
2550/03 20130101; F01N 3/0885 20130101; Y02C 20/10 20130101; F01N
2610/03 20130101; F01N 2570/04 20130101; F02B 37/00 20130101; B01D
53/9495 20130101; F01N 11/007 20130101; F01N 2250/12 20130101; F01N
3/0842 20130101; F02D 41/1454 20130101; F01N 11/002 20130101; F02D
41/028 20130101; F01N 2430/00 20130101 |
Class at
Publication: |
423/239.1 ;
423/213.2; 422/177; 060/274; 060/277; 060/284 |
International
Class: |
B01J 008/02; C01B
021/00; C01B 023/00; C01B 025/00; C01G 028/00; C01G 030/00; C01B
031/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2003 |
JP |
JP2003-376016 |
Claims
What is claimed is:
1. A sulfur purge control method, using an exhaust gas purifying
system for removing NOx by a NOx occlusion reduction type catalyst
from the exhaust gas of an internal combustion engine, said exhaust
gas purifying system having; an oxygen concentration detection
means for detecting the oxygen concentration in the downstream of
the NOx occlusion reduction type catalyst; a sulfur purge start
judgment means; and a sulfur purge control means accompanying the
control of the air-fuel ratio state of the exhaust gas; said sulfur
purge control method comprising steps of: controlling the air-fuel
ratio in the exhaust gas by setting a target air-fuel ratio to a
predetermined first air-fuel ratio which is a rich air-fuel ratio
by means of the sulfur purge control means after starting the
sulfur purge; and thereafter, controlling the air-fuel ratio in the
exhaust gas by changing the target air-fuel ratio to a
predetermined second air-fuel ratio which is a stoichiometric
air-fuel ratio, when the oxygen concentration measured by an oxygen
concentration detection means in the downstream of a NOx occlusion
reduction type catalyst becomes lower than a predetermined
threshold.
2. The sulfur purge control method of claim 1, wherein: the
predetermined first air-fuel ratio is between 0.93 and 0.98 which
is converted to the excess air factor; and the predetermined second
air-fuel ratio is between 0.997 and 1.002 which is converted to the
excess air factor.
3. An exhaust gas purifying system for removing NOx by a NOx
occlusion reduction type catalyst from the exhaust gas of an
internal combustion engine, comprising: an oxygen concentration
detection means for detecting the oxygen concentration in the
downstream of the NOx occlusion reduction type catalyst; a sulfur
purge start judgment means; and a sulfur purge control means
accompanying the control of the air-fuel ratio state of the exhaust
gas, wherein said sulfur purge control means includes; a first
sulfur purge control means for controlling the air-fuel ratio state
in the exhaust gas targeting a predetermined first air-fuel ratio
which is a rich air-fuel ratio; and a second sulfur purge control
means for controlling the air-fuel ratio state in the exhaust gas
targeting a predetermined second air-fuel ratio which is a
stoichiometric air-fuel ratio, and wherein after the start of
sulfur purge, the sulfur purge is controlled by said first sulfur
purge control means when the oxygen concentration in the downstream
of the NOx occlusion reduction type catalyst measured by said oxide
concentration detection means is a predetermined threshold or more;
and the sulfur purge is controlled by said second sulfur purge
control means when it is lower than the predetermined
threshold.
4. The exhaust gas purifying system of claim 3, wherein the
predetermined first air-fuel ratio is between 0.93 and 0.98 which
is converted to the excess air factor; and the predetermined second
air-fuel ratio is between 0.997 and 1.002 which is converted to the
excess air factor.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention concerns an exhaust gas purifying
system provided with a NOx occlusion reduction type catalyst for
removing NOx in the exhaust gas of an internal combustion engine
such as a diesel engine and a sulfur purge control method in the
exhaust gas purifying system.
[0002] Various researches and proposals have been made concerning
the catalyst type exhaust gas purifying system for reducing and
removing NOx from the exhaust gas of an internal combustion engine
of an automobile, a floor type internal combustion engine and the
like. In particular, NOx reduction type catalysts and
three-way-catalysts are used for purifying the exhaust gas of an
automobile and the like.
[0003] NOx occlusion reduction type catalyst is one such catalyst.
This catalyst fulfils its ability of NOx occlusion or its ability
of NOx release and removal based on the oxygen concentration in the
exhaust gas. For this catalyst, a porous catalyst coat layer of
alumina (Al.sub.2O.sub.3) or the like supports a catalyst metal
that oxidizes NOx and a NOx occluding material that occludes NOx.
As for this catalyst metal, platinum (Pt), palladium (Pd), or the
like can be utilized. On the other hand, the NOx occluding material
is composed of any one or several in combination of alkali metals,
alkaline-earth metals, rare-earths and the like. These alkali
metals include sodium (Na), potassium (K), cesium (Cs) and the
like. The alkaline-earth metals include calcium (Ca), barium (Ba)
and the like, while the rare-earths include yttrium (Y), lanthanum,
and the like.
[0004] Now, NOx removal by the above described NOx occlusion
reduction type catalyst will be explained.
[0005] In an exhaust gas condition in which the oxygen
concentration in the exhaust gas is high (lean air/fuel ratio
state) as in the normal driving state of diesel engine, lean-burn
gasoline engine and the like, the exhaust gas is cleaned as shown
in FIG. 4. Nitrogen monoxide (NO) to be discharged is oxidized with
oxygen (O.sub.2) which is present in the exhaust gas by the
oxidizing ability of a catalyst metal 21 and 22, and becomes
nitrogen dioxide (NO.sub.2). Next, this nitrogen dioxide (NO.sub.2)
is occluded in a NOx occluding material 23 in the form of nitrate.
As a result, the exhaust gas is cleaned.
[0006] However, when this occlusion of NOx continues, the NOx
occluding material 23 such as barium transforms into nitride and is
gradually saturated. Consequently, the NOx occluding material 23
loses its ability to occlude nitrogen dioxide (NO.sub.2).
Therefore, driving conditions of an engine are changed and the
rich-burn is performed generating exhaust gas (rich spike gas) of
low oxygen concentration, high carbon monoxide concentration, and
high exhaust temperature and delivering the gas to the
catalyst.
[0007] In this rich air-fuel ratio state of the exhaust gas, the
NOx occluding material 23, which occluded nitrogen dioxide
(NO.sub.2) and changed into nitride, releases the nitrogen dioxide
(NO.sub.2) that it has occluded and returns to the original barium
(Ba) and the like, as shown in FIG. 5. As oxygen (O.sub.2) is
absent in the exhaust gas, this released nitrogen dioxide
(NO.sub.2) is reduced on the catalyst metal using carbon monoxide
(CO), hydrocarbon (HC) and hydrogen (H.sub.2) in the exhaust gas as
reducer. As a result, nitrogen dioxide (NO.sub.2) is transformed
into nitrogen (N.sub.2), water (H.sub.2O), and carbon dioxide
(CO.sub.2) and cleaned.
[0008] In this NOx occlusion reduction type catalyst, however,
there is a problem that the NOx purifying efficiency falls as
driving continues because sulfur (sulfur component) in the fuel is
accumulated in the NOx occluding material in the catalyst.
Consequently, as described in Japanese Patent Laid-Open
1998-274031, it is necessary to perform substantially periodical
sulfur purges (sulfur component desorption) by setting the
temperature of the exhaust gas flowing into the catalyst at
approximately 600.degree. C. to 650.degree. C. or more and in rich
atmosphere, though different depending on catalysts.
[0009] In this sulfur purge, there is a problem that carbon
monoxide (CO) is discharged outside the engine.
[0010] In other words, in this sulfur purge, sulfur (S) is absorbed
in the NOx occluding material in the form of nitride. Consequently,
sulfur component (S) is released as sulfur dioxide (SO.sub.2), by
transforming sulfate into carbonate with carbon monoxide (CO), in
an oxygen-free and high temperature state. For this reason, the
oxygen-free and high temperature state is realized by putting the
exhaust gas in a rich air-fuel ratio state and by raising the
temperature of the catalyst in case of diesel engines. This rich
air-fuel ratio state is realized by reducing the exhaust quantity
through intake throttling, large quantity of EGR and the like, and
by performing post-injection, direct gas oil addition to the
exhaust pipe, and the like. In addition, the temperature rise of
the catalyst is realized by heating the catalyst with the heat
generated by the oxidation of added fuel through the catalytic
function.
[0011] In this hot rich atmosphere, carbon monoxide (CO) is
produced by partial decomposition of hydrocarbon (HC), fuel;
namely, through combustion of hydrogen component. On the other
hand, in the NOx occluding material, nitrogen dioxide (NO.sub.2) is
released more actively than sulfur dioxide (SO.sub.2) because
nitride reacts more with carbon monoxide (CO) and changes into
carbonate, compared to sulfate.
[0012] For this reason, in the prophase of the rich air-fuel ratio
state performed by this sulfur purge control, reactions as follow
will occur. Though sulfur dioxide (SO.sub.2) is released from the
NOx occluding material 23, nitrogen dioxide (NO.sub.2) is released
more actively. As a result, as shown in FIG. 5, carbon monoxide
(CO) is used for reduction and removal of NOx to be released. In
addition, carbon monoxide (CO) reacts with oxygen (O.sub.2)
released by reduction of this nitrogen dioxide (NO.sub.2). Hence,
carbon monoxide (CO) is not discharged outside the engine.
[0013] However, in the later stage of the rich air-fuel ratio state
as the sulfur purge progresses, reactions as follow will occur.
Though the release of sulfur dioxide (SO.sub.2) from the NOx
occluding material 23 is sustained, the release of nitrogen dioxide
(NO.sub.2) almost terminates in the latter stage of the rich
air-fuel ratio state. As a result, as shown in FIG. 6, carbon
monoxide (CO) is no longer used for reduction of nitrogen dioxide
(NO.sub.2), and oxygen (O.sub.2) released by this reduction also
decreases. Then, the oxygen concentration falls rapidly and the
carbon monoxide concentration increases. As a result, carbon
monoxide (CO) is discharged outside the engine.
[0014] FIG. 7 schematically shows the circumstances of this rapid
decrease of oxygen concentration by the upstream excess air factor
.lambda.(u) and the downstream excess air factor .lambda.(d) of the
NOx occlusion reduction type catalyst. FIG. 8 shows examples of
time series of upstream oxygen concentration (O.sub.2(u)) and
downstream oxygen concentration (O.sub.2(d)) of the NOx occlusion
reduction type catalyst, sulfur dioxide (SO.sub.2), and carbon
monoxide (CO), in the sulfur purge of the prior art. It can be
observed in the T1 portion indicated by an arrow that the upstream
oxygen concentration (O.sub.2(u)) decreases rapidly and carbon
monoxide (CO) increases.
BRIEF SUMMARY OF THE INVENTION
[0015] The present invention has an object of providing a sulfur
purge control method permitting to purge efficiently sulfur
component accumulated in a NOx occlusion reduction type catalyst,
all the way preventing discharge of carbon monoxide into the
atmospheric air in an exhaust gas purifying system for removing NOx
by the NOx occlusion reduction type catalyst and the exhaust gas
purifying system.
[0016] The aforementioned object can be attained by a sulfur purge
control method comprising steps of;
[0017] controlling the air-fuel ratio in an exhaust gas by setting
the target air-fuel ratio at a predetermined first air-fuel ratio
which is a rich air-fuel ratio by means of a sulfur purge control
means, after starting the sulfur purge; and
[0018] thereafter, controlling the air-fuel ratio in the exhaust
gas by changing the target air-fuel ratio to a predetermined second
air-fuel ratio which is a stoichiometric air-fuel ratio, when the
oxygen concentration downstream a NOx occlusion reduction type
catalyst measured by an oxygen concentration detection means
becomes lower than a predetermined threshold,
[0019] in an exhaust gas purifying system for removing NOx by the
NOx occlusion reduction type catalyst from the exhaust gas of an
internal combustion engine, the system comprising:
[0020] the oxygen concentration detection means for detecting the
oxygen concentration downstream the NOx occlusion reduction type
catalyst; a sulfur purge start judgment means; and
[0021] the sulfur purge control means accompanying the control of
exhaust gas air-fuel ratio state.
[0022] According to this sulfur purge control method, as the oxygen
concentration in the exhaust gas increases in the sulfur purge
after the termination of release of NOx from a NOx occluding
material, the sulfur purge can be made more efficient, all the way
preventing carbon monoxide from being emitted into the atmospheric
air.
[0023] Here, in the aforementioned sulfur purge control method, if
it is so composed that said predetermined first air-fuel ratio be
between 0.93 and 0.98 which is converted to the excess air factor
and said predetermined second air-fuel ratio be between 0.997 and
1.002 which is converted to the excess air factor, the sulfur purge
becomes more efficient, all the way preventing more carbon monoxide
from being emitted into the atmospheric air.
[0024] In addition, the exhaust gas purifying system for attaining
the aforementioned object is an exhaust gas purifying system for
removing NOx by the NOx occlusion reduction type catalyst from the
exhaust gas of an internal combustion engine, comprising:
[0025] an oxygen concentration detection means for detecting the
oxygen concentration downstream the NOx occlusion reduction type
catalyst;
[0026] a sulfur purge start judgment means; and
[0027] a sulfur purge control means accompanying the control of
exhaust gas air-fuel ratio state, wherein
[0028] said sulfur purge control means includes a first sulfur
purge control means for controlling the air-fuel ratio state in the
exhaust gas targeting a predetermined first air-fuel ratio which is
a rich air-fuel ratio; and
[0029] a second sulfur purge control means for controlling the
air-fuel ratio state in the exhaust gas targeting a predetermined
second air-fuel ratio which is a stoichiometric air-fuel ratio, and
wherein
[0030] the sulfur purge is controlled by said first sulfur purge
control means when the oxygen concentration downstream the NOx
occlusion reduction type catalyst measured by said oxide
concentration detection means is equal or superior to a
predetermined threshold, after the start of sulfur purge, while the
sulfur purge is controlled by said second sulfur purge control
means when lower than the predetermined threshold.
[0031] According to this exhaust gas purifying system, the
aforementioned sulfur purge control method can be performed. As the
oxygen concentration in the exhaust gas increases in the sulfur
purge after the termination of release of NOx from a NOx occluding
material, the sulfur purge can be made more efficient, all the way
preventing carbon monoxide from being emitted into the atmospheric
air.
[0032] Then, in the aforementioned exhaust gas purifying system, if
it is so composed that said predetermined first air-fuel ratio be
between 0.93 and 0.98 which is converted to the excess air factor
and said predetermined second air-fuel ratio be between 0.997 and
1.002 which is converted to the excess air factor, the sulfur purge
becomes more efficient, all the way preventing more carbon monoxide
from being emitted into the atmospheric air.
[0033] According to the sulfur purge control method and the exhaust
gas purifying system of the present invention, the sulfur purge can
be made more efficient, all the way preventing carbon monoxide from
being emitted into the atmospheric air, even after the termination
of release of NOx from a NOx occluding material, because the
air-fuel ratio in the exhaust gas is controlled by changing the
target air-fuel ratio from the predetermined first air-fuel ratio
which is a rich air-fuel ratio to a predetermined second air-fuel
ratio which is a stoichiometric air-fuel ratio, when the oxygen
concentration downstream a NOx occlusion reduction type catalyst
becomes lower than a predetermined threshold, after the start of
sulfur purge.
[0034] In other words, the oxygen concentration in the exhaust gas
flowing in the NOx occlusion reduction type catalyst is raised by
changing the air-fuel ratio state of the exhaust gas in the sulfur
purge control of the NOx occlusion reduction type catalyst
according to the variation of oxygen concentration, through the
monitoring of the oxygen concentration downstream a NOx occlusion
reduction type catalyst. Therefore, carbon monoxide production can
be limited and, at the same time, the produced carbon monoxide can
be oxidized and the emission of carbon monoxide into the
atmospheric air can be prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 shows a composition of an exhaust gas purifying
system of an embodiment according to the present invention;
[0036] FIG. 2 shows a composition of a control means of the exhaust
gas purifying system of the embodiment according to the present
invention;
[0037] FIG. 3 shows an example of control flow for sulfur purge of
the embodiment according to the present invention;
[0038] FIG. 4 shows schematically reactions in the lean air-fuel
ratio state in a NOx occlusion reduction type catalyst;
[0039] FIG. 5 shows schematically reactions in the prophase of rich
air-fuel ratio state in the NOx occlusion reduction type
catalyst;
[0040] FIG. 6 shows schematically reactions in the anaphase of rich
air-fuel ratio state in the NOx occlusion reduction type
catalyst;
[0041] FIG. 7 shows schematically the temporal variation of oxygen
concentration in a sulfur purge of the prior art; and
[0042] FIG. 8 shows an example of the time series of oxygen
concentration, sulfur dioxide concentration monoxide concentration
in the sulfur purge of the prior art.
DETAILED DESCRIPTION OF THE INVENTION
[0043] The sulfur purge control method and the exhaust gas
purifying system of the embodiment according to the present
invention shall be described referring to the drawings.
[0044] FIG. 1 shows a composition of an exhaust gas purifying
system 1 of an embodiment according to the present invention. For
this exhaust gas purifying system 1, an emission control device 10
having a NOx occlusion reduction type catalyst converter 11
provided with a NOx occlusion reduction type catalyst is disposed
in an exhaust passage 4 of an engine (internal combustion
engine).
[0045] This NOx occlusion reduction type catalyst converter 11 is
formed with monolithic catalyst. In this monolithic catalyst, a
catalyst coat layer is deposited on a support body of aluminum
oxide, titan oxide and the like. This catalyst coat layer is made
to support a catalytic metal such as platinum (Pt) and palladium
(Pd), and a NOx occluding material (NOx occluding substance) such
as barium (Ba).
[0046] In this NOx occlusion reduction type catalyst converter 11,
NOx is prevented from flowing into the atmospheric air in the
following manner. NOx in the exhaust gas is removed by occluding
NOx with the NOx occluding material, when the oxygen concentration
of the exhaust gas is high (lean air-fuel ratio state). On the
other hand, when the oxygen concentration of the exhaust gas is low
or null (rich air-fuel ratio state), NOx is prevented from flowing
into the atmospheric air by releasing the occluded NOx, and
reducing the released NOx through catalytic action of the catalytic
metal.
[0047] A first oxygen concentration sensor 13 and a second oxygen
concentration sensor 14 are respectively disposed in the upstream
and down stream, namely, in front of and behind this NOx occlusion
reduction type catalyst 11. As these first and second oxygen
concentration sensors 13 and 14, a sensor detecting only the oxygen
concentration may be used; however, a sensor integrating a .lambda.
sensor (excess air factor sensor), a NOx concentration sensor and
an oxygen concentration sensor can be used.
[0048] In addition, in order to judge the temperature of the NOx
occlusion reduction type catalyst 11, a first temperature sensor 15
and a second temperature sensor 16 are disposed respectively
upstream and down stream, namely, in front of and behind the NOx
occlusion reduction type catalyst 11.
[0049] Moreover, an HC supply valve 12 for supplying gas oil or
other fuels that becomes reducer of NOx, namely hydrocarbon (HC),
is installed in an exhaust passage 4 in the upstream of the NOx
occlusion reduction type catalyst 11. This HC supply valve 12
injects hydrocarbon (HC) which is a fuel such as gas oil from a
fuel tank that is not illustrated directly into the exhaust passage
4. This HC supply valve 12 composes an air-fuel ratio control means
for making the air-fuel ratio of exhaust gas in rich state or in
stoichiometric state (theoretical air-fuel ratio state). It should
be appreciated that the arrangement of this HC supply valve 12 may
be omitted, in the case where a similar air-fuel ratio control is
performed by the post-injection during fuel injection in the
cylinder of an engine E.
[0050] Then, a control unit (ECU: engine control unit) 20 is
installed for performing a general control of the driving of the
engine E and for performing the regeneration control of NOx removal
capacity of the NOx occlusion reduction type catalyst converter 11.
Detection values from the first as well as second oxygen
concentration sensors 13 and 14, the first as well as the second
temperature sensors 15 and 16 and the like are input in this
control unit 20. Signals for controlling an EGR valve 6 of the
engine E, a fuel injector 8 of a common-rail electric control fuel
injection system for fuel injection as well as an intake throttle
9, and the like are output from this control unit 20.
[0051] In this exhaust gas purifying system 1, air A, exhaust gas G
and EGR gas Ge flow as follow. The air A passes through a
compressor 3a of a turbo-charger 3 and the intake throttle (intake
throttle valve) 9 of an intake passage 2, and enters the cylinder
from an intake manifold 2a. Then, the quantity of the air A is
adjusted by the intake throttle (intake throttle valve) 9. On the
other hand, the exhaust gas G generated in the cylinder is
discharged out of an exhaust manifold 4a into the exhaust passage
4, drives a turbine 3b of the turbo-charger, passes through the
emission control device 10, and is discharged out into the
atmospheric air through a silencer that is not illustrated. This
exhaust gas G becomes exhaust gas Gc that is purified by the
emission control device 10. In addition, a part of exhaust gas G
passes through an EGR cooler 7 and an EGR valve 6 of an EGR passage
5 as EGR gas Ge, and recirculates to the intake manifold 2a. This
EGR gas Ge is adjusted in terms of its quantity by the EGR valve
6.
[0052] Then, the control unit of the exhaust gas purifying system 1
is built in a control unit 20 of the engine E. This control unit 20
performs the driving control of the engine E and the control of the
exhaust gas purifying system 1. As shown in FIG. 2, the control
unit of this exhaust gas purifying system 1 is composed by
providing a control means C1 of exhaust gas purifying system 1 that
has an exhaust gas composition detection means C10, a control means
C20 of NOx occlusion reduction type catalyst and the like.
[0053] The exhaust gas composition detection means C10 is composed
by having a NOx concentration detection means C11 and an oxygen
concentration detection means C12. This means C10 is the means for
detecting NOx concentration and oxygen concentration in the exhaust
gas. The NOx concentration detection means C11 is composed of a NOx
concentration sensor and the like that are not illustrated. The
oxygen concentration detection means C12 is composed of the first
and second oxygen concentration sensors 13 and 14 and the like. It
should be appreciated that, the NOx concentration detection means
C11 and the oxygen concentration detection means C12 jointly use
this sensor, in case of using a sensor integrating a .lambda.
sensor, a NOx concentration sensor and an oxygen concentration
sensor.
[0054] The control means C20 of NOx occlusion reduction type
catalyst is means for performing the regeneration of the NOx
occlusion reduction type catalyst converter 11 and the control of
sulfur purge and the like. This means C20 is composed by having a
regeneration start judgment means C21 of NOx catalyst, a
regeneration control means C22 of NOx catalyst, a sulfur purge
start judgment means C23, a sulfur purge control means C24 and the
like.
[0055] The regeneration start judgment means C21 of NOx catalyst
calculates the NOx removal rate from NOx concentrations upstream
and downstream the NOx occlusion reduction type catalyst converter
11 and judges to start the regeneration of the NOx catalyst in case
where this NOx removal rate becomes lower than a predetermined
threshold. These NOx concentrations are detected, for instance, by
the NOx concentration detection means C11.
[0056] In addition, the regeneration control means C22 of NOx
catalyst sets the exhaust gas state to a predetermined rich
air-fuel ratio state and a predetermined temperature range
(approximately 200.degree. C. to 600.degree. C. depending on
catalysts), by the post-injection or injection into the exhaust
pipe in the fuel injection control of the engine E, EGR control,
intake throttling control and the like. Thereby, the NOx removal
performance, namely NOx occluding capacity is restored to
regenerate the NOx catalyst.
[0057] In addition, the sulfur purge start judgment means C23 is
the means for deciding to start the sulfur purge control or not
based on whether the sulfur has accumulated enough to deteriorate
the NOx occluding capacity or not that is determined by multiplying
the sulfur accumulation quantity Sa or by other methods. This means
C23 decides to start the sulfur purge when the sulfur accumulation
quantity Sa becomes equal or superior to the predetermined
threshold Sa0.
[0058] The sulfur purge control means C24 is composed by having a
catalyst temperature rise control means C241, a first sulfur purge
means C242 and a second sulfur purge means C243. This means C24
performs the sulfur purge efficiently, while limiting emission of
carbon monoxide (CO) into the atmospheric air. This catalyst
temperature rise control means C241 is a means for raising the
temperature of NOx occlusion reduction type catalyst until the
sulfur purge becomes possible, by controlling the exhaust gas
air-fuel ratio through post-injection or injection in the exhaust
pipe, and by performing EGR control and intake throttling control.
In addition, the first sulfur purge means C242 is a means for
performing the sulfur purge by setting a first air-fuel ratio
(.lambda.=0.93 to 0.98 which is converted to excess air factor) to
the target air-fuel ratio. The second sulfur purge means C243 is a
means for performing the sulfur purge by setting a second air-fuel
ratio (.lambda.=0.997 to 1.002 which is converted to excess air
factor) to the target air-fuel ratio.
[0059] The sulfur purge control method of the NOx occlusion
reduction type catalyst according to the present invention in the
exhaust gas purifying system 1 provided with these control means of
exhaust gas purifying system is performed according to a control
flow for sulfur purge as illustrated in FIG. 3.
[0060] This control flow in FIG. 3 is shown as in the following
control flow. Namely, this control flow is called up repeatedly
from the control flow of the entire control for the exhaust gas
purifying system 1, together with the control flow concerning the
regeneration of NOx occluding capacity of the NOx occlusion
reduction type catalyst converter 11 and the like. This control
flow judges if the sulfur purge is necessary or not, and performs
the sulfur purge control when it is necessary.
[0061] When this control flow starts, in the step S10, sulfur
accumulation quantity Sa occluded and accumulated by the NOx
occlusion reduction type catalyst of the NOx occlusion reduction
type catalyst converter 11 is calculated based on fuel consumption
and the sulfur quantity contained in the fuel.
[0062] In the next step S11, it is judged by the sulfur purge start
judgment means C23, whether or not to start the sulfur purge. In
this judgment, the sulfur purge is started in the case where the
sulfur accumulation quantity Sa becomes equal or superior to a
predetermined limit value Sa0. In the case where it is judged not
to start the sulfur purge in the judgment of the step S11, the
control flow for this sulfur purge is terminated to return. If it
is judged as the start of the sulfur purge, it proceeds to the step
S12.
[0063] In the check of catalyst temperature of this step S12, the
catalyst temperature Tc is calculated based on temperatures
detected by the first temperature sensor 15 and the second
temperature sensor 16 and it is judged if this catalyst temperature
Tc is equal or superior to a predetermined judgment temperature
(threshold) Tc. This predetermined judgment temperature is
approximately in the range of 600.degree. C. to 650.degree. C.,
depending on catalysts, and shall be the value of the temperature
at which sulfur is purged. The value is determined previously by
tests. In case where this catalyst temperature Tc is equal or
superior to the predetermined judgment temperature Tc0, it proceeds
to the step S14 as it is. However, if this catalyst temperature Tc
is lower than the predetermined judgment temperature Tc0, the
sulfur purge can not be performed efficiently. Therefore, in case
where this catalyst temperature Tc is judged to be lower than the
predetermined judgment temperature Tc0, the catalyst temperature
rise control is performed by the catalyst temperature rise control
means C241 in the step S13, until the catalyst temperature Tc
becomes equal or superior to the predetermined judgment temperature
Tc0. Thereafter, it proceeds to the step S14. More particularly,
after having performed the catalyst temperature rise control of the
step S13 for a predetermined period of time, the catalyst
temperature check of the step S12 is repeated. This predetermined
period of time is a time concerning the interval for checking the
catalyst temperature.
[0064] In this catalyst temperature rise control, post-injection in
the cylinder of the engine E is performed by a fuel injector 8, or
injection in the exhaust pipe is performed by directly injecting HC
which is fuel such as gas oil from an HC supply valve 12 to the
exhaust passage 4. Thereby, HC is activated on the NOx occlusion
reduction type catalyst and the temperature of this catalyst is
raised by its oxidation heat. It should be appreciated that EGR
control and intake throttling control are also performed in
parallel.
[0065] In the following steps S14 to S18, the sulfur purge control
is performed. First of all, the oxygen concentration is checked in
the step S14, to judge if NOx is released from NOx occlusion
reduction type catalyst.
[0066] In the course of this oxygen concentration check, if the
downstream oxygen concentration Od that is detected by the second
oxygen concentration sensor 14 is equal or superior to a
predetermined threshold Od0 (for instance, about 0 to 0.2%, or
about 0.997 to 1.002 which is converted to excess air factor
(.lambda.)), it is judged that the sulfur purge is in the prophase
or early stages in which NOx is released. The first air-fuel ratio
of .lambda.=0.93 to 0.98 is set as the target air-fuel ratio, by
the first sulfur purge means C242 when converted to excess air
factor until the downstream oxygen concentration Od becomes lower
than the predetermined threshold Od0, and the first sulfur purge
control is performed. In this control, the exhaust gas flowing in
the NOx occlusion reduction type catalyst converter 11 is set to a
fuel excessive rich air-fuel ratio (air-fuel ratio lower than the
theoretical air-fuel ratio) through a feedback control of the
upstream oxygen concentration to be detected by the first oxygen
concentration sensor 13 and the like, while performing EGR control
and intake valve (intake throttle) control for reducing the exhaust
gas flow in parallel. More particularly, after having performed the
first sulfur purge control of the step S15 for a predetermined
period of time, the oxygen concentration check of the step S14 is
repeated. This predetermined period of time is a time concerning
the interval for checking the oxygen concentration.
[0067] In this first sulfur purge control, as the NOx occlusion
reduction type catalyst becomes oxygen-free and high temperature
state, sulfur that has been occluded in the form of sulfate on the
occluding material of the high temperature NOx occlusion reduction
type catalyst is released in the form of sulfur dioxide (SO.sub.2).
On the other hand, NOx that has been occluded simultaneously in the
form of nitride is released in the form of nitrogen dioxide
(NO.sub.2). This nitrogen dioxide (NO.sub.2) is reduced by the
catalytic action of a precious metal oxidation catalyst, becomes
nitrogen (N.sub.2), and at the same time generates oxygen
(O.sub.2). Carbon monoxide (CO) in the exhaust gas is oxidized and
becomes harmless, and is released into the atmospheric air in the
form of carbon dioxide (CO.sub.2).
[0068] Hence, in the sulfur purge during which this NOx is being
released, release of carbon monoxide (CO) into the atmospheric air
(CO slip) hardly occurs. In addition, the downstream oxygen
concentration Od, which is detected by the second oxygen
concentration sensor 14 in the downstream of the NOx occlusion
reduction type catalyst converter 11, becomes equal or superior to
the predetermined threshold Od0 by oxygen (O.sub.2) that is
generated by reduction of nitrogen dioxide (NO.sub.2).
[0069] The reaction of the release of NOx that has been occluded in
the form of nitride occurs more easily than the release of sulfur
that has been occluded in the form of sulfate on the occluding
material of the NOx occlusion reduction type catalyst. Therefore,
release of NOx terminates before the completion of the sulfur
purge, as the sulfur purge progresses. As this NOx release
approaches its termination, the production of oxygen (O.sub.2) also
decreases. Consequently, if this first sulfur purge control is
sustained, carbon monoxide (CO) in the exhaust gas can not be
oxidized completely, and the quantity of carbon monoxide (CO) that
is released into the atmospheric air increases, provoking CO
slip.
[0070] In order to avoid the generation of this CO slip, in the
present invention, the downstream oxygen concentration Od is
monitored, and the oxygen concentration is checked in the step S14.
It is judged that the sulfur purge has entered the anaphase or
termination stage where the release of NOx has stopped, in case the
downstream oxygen concentration Od begins to fall suddenly and
becomes lower than the predetermined threshold Od0. Then, the first
sulfur purge control is terminated to switch over to the second
sulfur purge control of the step S18.
[0071] In this second sulfur purge control, the second air-fuel
ratio of .lambda.=0.997 to 1.002 is set as the target air-fuel
ratio when converted to excess air factor by the second sulfur
purge means C243 until it is judged that the sulfur purge in the
step S17 has been terminated, and the second sulfur purge control
is performed. In this control, the air-fuel ratio of the exhaust
gas flowing in the NOx occlusion reduction type catalyst converter
11 is set to a stoichiometric air-fuel ratio (theoretical air-fuel
ratio) through feedback control of the upstream oxygen
concentration to be detected by the first oxygen concentration
sensor 13 and the like, while performing EGR control or intake
valve control for reducing the exhaust gas flow in parallel,
similarly to the first sulfur purge control. More particularly,
after having performed the second sulfur purge control of the step
S18 for a predetermined period of time, the calculation of sulfur
desorption quantity integrated value Sp of the step 16 and the
check of termination of the sulfur purge of the step S17 are
repeated. This predetermined period of time is a time concerning
the interval for checking the sulfur purge termination.
[0072] Little oxygen (O.sub.2 remains in the exhaust gas of the NOx
occlusion reduction type catalyst converter 11 through the rise of
the upstream oxygen concentration Ou, by setting this target
air-fuel ratio from the first air-fuel ratio to the second air-fuel
ratio and by setting the air-fuel ratio state of the exhaust gas
flowing in the NOx occlusion reduction type catalyst converter 11
from the rich air-fuel ratio state to the stoichiometric air-fuel
ratio state. Hence, carbon monoxide (CO) in the exhaust gas is
oxidized and becomes harmless by this oxygen (O.sub.2), and is
released in the atmospheric air with the form of carbon dioxide
(CO.sub.2). Consequently, it is possible to sustain the sulfur
purge, while preventing CO slip, even after the termination of the
NOx release.
[0073] The judgment of the sulfur purge termination in the step S17
is performed by judging if the sulfur desorption quantity
integrated value Sp calculated in the step 16 (sulfur release
quantity integrated value) is equal or superior to the sulfur
accumulation quantity Sa calculated in the step S10. This sulfur
desorption quantity integrated value Sp is determined by
integrating desulfurization quantity obtained by referring to
engine driving state, and desulfurization quantity map inputted
beforehand and the like, based on the temperature to be detected by
the first and the second temperature sensors 15 and 16. The sulfur
purge is judged to have terminated and the second sulfur purge
control is terminated to return, in the case where the sulfur
desorption quantity integrated value Sp becomes equal or superior
to the sulfur accumulation quantity Sa, in the judgment of the step
S17. It should be appreciated that, it proceeds to the step 18 if
the sulfur desorption quantity integrated value Sp is not equal or
not superior to the sulfur accumulation quantity Sa in the judgment
of the step S17. Then, in the step S18, after having performed the
second sulfur purge for a predetermined period of time, it returns
to the step S16, for integrating the sulfur desorption quantity
integrated value Sp. Then, the judgment of the step S17 is
repeated.
[0074] According to the aforementioned sulfur purge control and
exhaust gas purifying system 1, the sulfur purge can be performed
efficiently, while preventing carbon monoxide (CO) from leaking
into the atmospheric air, during the sulfur purge, and especially
during the sulfur purge anaphase in which the release of NOx has
terminated.
[0075] In the aforementioned composition, the emission control
device 10 was described to be composed only of the NOx occlusion
reduction type catalyst converter 11; however, the present
invention can also be applied to the cases where it is composed by
combining with a diesel particulate filter (DPF) that is formed as
a separate body, or where it is composed by supporting the NOx
occlusion reduction type catalyst by the DPF.
* * * * *